1. Field of Invention
The invention relates to a frequency synthesizing device and, in particular, to a frequency synthesizing device and method with automatic calibration.
2. Related Art
The oscillating frequency of a voltage-controlled oscillator (VCO) is determined by inductors and capacitors connected in parallel. One can change the oscillating frequency of the VCO by adjusting the capacitance.
FIG. 1 shows a frequency synthesizer in the prior art. The frequency synthesizer includes a VCO 120, a frequency divider device 200, a frequency phase detector 300, and a low-pass filter 190. The VCO 120 generates an output signal. The frequency divider device 200 removes the frequency of the input signal and produces a feedback signal. The phase detector 300 compares the frequencies and phases of the input signal and the feedback signal. If the frequency of the input signal is higher than that of the feedback signal, the control voltage of the VCO 120 is raised; otherwise, the control voltage is lowered.
Since the VCO has many bands. The control method is done by digital signal rough-tuning, setting the working band of the VCO. The fine-tuning is done with analog signals, locking the output signal at a predetermined frequency.
The frequency synthesizing device proposed in the U.S. Pat. No. 6,731,176 sets the VCO in an initial band and uses a frequency locking detector to detect whether the frequency locking is successful. If the frequency locking is unsuccessful for a while, then the VCO is adjusted to an adjacent band to start frequency locking again until it is successful. The VCO is then set at that band. Since the frequency locking detector operates at the comparison frequency, its speed is slower.
The frequency synthesizing device proposed in the U.S. Pat. No. 6,747,519 performs rough tuning using a comparison voltage. If after a period of time the control voltage of the VCO still exceeds a predetermined voltage range, it means the frequency locking is unsuccessful. The VCO is adjusted to an adjacent band to start frequency locking again until it is successful. The VCO is then set at that band. Such a device often has to try many bands before a frequency locking is successful. Moreover, comparing analog voltage signals may be easily affected by the variation in production process.
Through the two patents provide feasible frequency locking methods, it takes some time for the frequency synthesizer to lock a frequency in order to determine whether the selected band is correct. Thus, the rough tuning is completely after a longer time. This is unacceptable for systems that have stricter requirement in the frequency synthesizing time.